Changed CLI to use an argparse-based parser

[reklawnos]

From previous pull request that got reverted:

The "commands" file was pretty messy, so I put together an argparse parser for command line arguments. It'll handle improper number of arguments/optional arguments/help text for us. There is only one breaking change I noticed which is that listversions now takes a -d or --documents flag for showing document versions, so instead of ./freejournal_cli.py listversions documents, it's ./freejournal_cli.py listversions -d or ./freejournal_cli.py listversions --documents.

@pdaian, can you take a look at the help messages for the commands and make sure they look reasonable? Additionally, if naming is off for anything just let me know.